Figure 4.13 Desiccation resistance of arboreal and terrestrial ant species, measured as LT50 (h) for groups of 10-22 ants at a, = 0. LT50 is time to 50% mortality of the sample. The terrestrial group includes species from the Chihuahuan Desert. Note log scale. Source: Hood and Tschinkel (1990). Physiological Entomology 15, 23-35, Blackwell Publishing.

terrestrial ants of the same size (Fig. 4.13). This may be due to better waterproofing by epicuticular lipids and more efficient extraction of faecal water, but is not due to differences in water loss tolerance. Arboreal ants lack the considerable microclimatic advantages of underground nests, in which av may approach saturation at depths of 50 cm (Hood and Tschinkel 1990). Two species of desert honeypot ants, Myrmecocystus, with polymorphic workers showing a great range of size, have water loss rates which increase as dry mass0.31, so that again larger ants lose water disproportionately more slowly (Duncan and Lighton 1994). High water contents (about 84 per cent) in Myrmecocystus workers may be related to their nectar diet and enable high desiccation tolerance. In harvester ants, Messor pergandei, with a body mass range of 1-12 mg, small workers lose water faster and have smaller water reserves than large workers, even after size correction, so their foraging times must be correspondingly reduced (Lighton et al. 1994). Finally, when populations of imported fire ants, Solenopsis invicta, were sampled across Texas, differences in desiccation resistance of minor workers were not correlated with head width (Phillips et al. 1996b). Duncan and Lighton (1994) give a good account of the significance of water relations for ant foraging strategies.

Broad-scale, multi-species assessments of physiological variation in beetles (macrophysiology) have demonstrated good correlations between habitat aridity and desiccation resistance. Comparison of six species of sub-Antarctic weevils (Coleoptera, Curculionidae) belonging to the Ectemnorhinus group of genera showed that species from moist habitats had relatively low water contents, high rates of water loss, and reached maximum tolerable water loss faster than those from dry rock face habitats (Chown 1993). Similar relationships are apparent in sub-Antarctic carabid and perimylopid beetles, in which physiological adaptations to restrict water loss are reduced in the smaller carabids, which compensate by inhabiting moister habitats (Todd and Block 1997). Southern African keratin beetles of the genera Trox and Omorgus (Trogidae) differ from many other beetles in showing little osmoregulatory ability, and the correlation between water loss rates and habitat aridity is partly due to variation in body size (Le Lagadec et al. 1998). These authors suggest that there may be strong selection for large body size in desert species. The partitioning of variance in desiccation resistance was also examined in trogids. Most variance in body size, and the physiological traits that are strongly influenced by body size (water and lipid content, maximum tolerable water loss, rate of water loss) is partitioned at the generic level (50-70 per cent), then at the species level (20-50 per cent) (Chown et al. 1999). This suggests a certain confidence in past investigations of insect water loss, especially those that provide mass-specific data.

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